Einstein Photoelectric Theory: Class 12 Physics Notes, Definition & Working Principle

The Einstein Photoelectric Theory was an early attempt to explain the photoelectric effect before Einstein’s quantum explanation. This theory was based on classical physics and assumed that light, as a continuous electromagnetic wave, gradually transferred energy to electrons on a metal surface.

Key Ideas of Electrostatic Photoelectric Theory

• The theory treated light as a continuous wave rather than a particle or photon.
• It suggested that electrons absorb energy over time from the incoming light. Once they gain enough energy, they escape from the metal surface.
• According to this idea, dimmer light (lower intensity) should cause a longer delay in electron emission because it transfers energy more slowly.
• In reality, electrons are emitted immediately when exposed to high-frequency light, even if it's very weak. Also, low-frequency light (even if intense) fails to eject electrons at all—this contradicted the classical theory.

As per NCERT: "In 1905, Albert Einstein (1879-1955) proposed a radically new picture of electromagnetic radiation to explain photoelectric effect. In this picture, photoelectric emission does not take place by continuous absorption of energy from radiation. Radiation energy is built up of discrete units – the so called quanta of energy of radiation. Each quantum of radiant energy has energy $h\nu$ , where h is Planck’s constant and n the frequency of light. In photoelectric effect, an electron absorbs a quantum of energy (hn ) of radiation. If this quantum of energy absorbed exceeds the minimum energy needed for the electron to escape from the metal surface (work function ${\phi }_0$ ), the electron is emitted with maximum kinetic energy $$K_{\max }=h\nu -{\phi }_0\left(11.2\right)$$

More tightly bound electrons will emerge with kinetic energies less than the maximum value. Note that the intensity of light of a given frequency is determined by the number of photons incident per second. Increasing the intensity will increase the number of emitted electrons per second. However, the maximum kinetic energy of the emitted photoelectrons is determined by the energy of each photon. Equation (11.2) is known as Einstein’s photoelectric equation."

Derivation of the Photoelectric Equation

In this section, we derive the core relation

$$K_{\max }=h\nu -{\phi }_0$$

which shows how a photon’s energy splits into the work needed to free an electron and the electron’s kinetic energy. Starting from the particle nature of light, we employ energy conservation, introduce the concept of work function, and connect to the experimentally measurable stopping potential. Through each step, we demonstrate why no classical wave description can account for the instantaneous emission or frequency dependence of the emitted electrons’ kinetic energy.

3.2.1 Photon Energy and the Work Function

Light of frequency ν\nuν is composed of quanta (photons), each carrying energy

$E_{\mathrm{photon}}=h\nu$

where hhh is Planck’s constant $\left(6.626\times {10}^{-34}\text{ J·s}\right)$ . Each photon interacts with a single electron in the metal, transferring its entire energy in one discrete event. To liberate an electron from the surface, a minimum energy—called the work function ${\varphi }_0$ }—must be supplied, which depends on the metal’s electronic binding and surface conditions. Any photon with $h\nu <{\varphi }_0$ ​ cannot free an electron, regardless of how intense the light is, because individual photons lack sufficient energy.

3.2.2 Conservation of Energy at the Metal Surface

When a photon of energy $h\nu$ is absorbed by an electron at the metal surface, energy conservation yields:

$\underset{\text{incoming photon energy}}{h\nu }=\underset{\text{energy to free electron}}{{\varphi }_0}+\underset{\text{kinetic energy of emitted electron}}{K_e}$

where $K_e$ ​ is the actual kinetic energy of a particular photoelectron. Because some electrons originate deeper in the metal and lose energy through collisions, the experimentally observed electrons exhibit a range of kinetic energies; the maximum kinetic energy $K_{\max }$ ​ corresponds to those emitted from the surface without additional losses. By isolating $K_{\max }$ ​, we rearrange:

$$K_{\max }=h\nu -{\phi }_0$$

 

This equation immediately explains why $K_{\max }$ ​ depends linearly on $\nu$ (slope h) and is zero at the threshold frequency ${\nu }_0=\frac{{\varphi }_0}{h}$

The following areas find application of Einstien's Photoelectric Theory:

1. Night-Vision Goggles and Low-Light Cameras

In a night-vision tube, each tiny bit of ambient light (even starlight) creates a photoelectron. That electron gets amplified until you see a bright, clear image on a screen. Modern astronomical cameras use back-illuminated CCDs to detect very faint optical or UV signals in space research.

2. Quantum Computing Readout Techniques

To read the state of a superconducting qubit or a trapped ion, researchers often use light. A single photon will cause an electron to jump or produce a tiny flash of light, indicating “0” or “1.” Detecting those electrons or photons reliably depends directly on the photoelectric principle.

3. Space Telescopes and Astronomical Cameras

Advanced CCDs in telescopes “eat” UV or X-ray photons and turn them into electrons. Even the faintest flicker of a distant star’s light becomes a measurable signal, allowing astronomers to explore galaxies billions of light-years away.